Direct conversion of solar energy to electrical energy can provide a virtually unlimited source of clean energy. Solar cells fabricated from semiconductor materials have been refined through years of research, and are commonly assembled in arrays to form photovoltaic modules for harvesting and converting solar energy to electrical energy. Conventional photovoltaic modules typically include a stack of materials including at least one encapsulation layer. The encapsulation layer is added to protect the photovoltaic module from the environment, and to provide electrical insulation of the module. One example of a conventional material often employed as an “encapsulant” for use in the encapsulation layer is ethylene vinyl acetate (EVA) copolymer.
The lamination process is an important step in the photovoltaic module manufacturing process, during which the silicon cells are laminated with one or more encapsulation layers and packaged in their final form before shipment. During an exemplary conventional lamination process, EVA is applied to a photovoltaic module and the EVA is cured so as to promote cross-linking to prevent creeping of the encapsulation layer(s) as a result of temperature or stress. Since the cross-linking reaction of the EVA is irreversible, cell reclamation is impossible should anything go wrong with the lamination/curing process. Therefore, extreme care is taken to ensure that the lamination conditions are set correctly in order to guarantee continuous production of reliable and durable modules, which meet international certification standards.
Improperly laminated modules often may develop defects leading to premature failures and the inevitable loss in module performance. For example, incompletely cured encapsulant retains its thermoplastic behavior, resulting in flowing or creeping when exposed to solar heat. Excessive flow may result in mechanical failure of the encapsulant, exposing the silicon cells to the outdoor environment, which could lead to electrical faults, cell or interconnection cracking, and corrosion. Additionally, poor adhesion or bubbles in the encapsulant may lead to optical losses in the solar module. On the other hand, EVA that is fully cured no longer flows, offers excellent resistance to creep, and ensures proper bonding of the encapsulant within the module.
The level of curing in EVA is correlated with the degree of polymer cross-linking that occurs during the lamination process. Two conventional methods commonly used for detection of the degree of cross-linking of EVA include the “gel fraction” test and the “creep” test. Unfortunately, both techniques involve destruction of the module. In the case of the gel fraction test, two days and wet chemistry lab capabilities are needed to complete the procedure. Another method for assessing polymer cross-linking includes a differential scanning calorimetry (DSC) technique developed by BP Solar. However, this technique offers limited resolution and has not been widely applied. Another limitation of the DSC technique is that it relies on knowledge of the thermal history of the sample (i.e. whether it had previously been cured, as well as the amount of time at elevated temperature). Furthermore, this technique also requires the destruction of a photovoltaic module to obtain the material needed for testing.